Existing-Facility Load by Peak Demand (NEC 220.87)
Adding load without a service upgrade, on the metered peak: NEC 220.87 lets an EXISTING building use the maximum demand the utility actually metered (a year of data, or a 30-day recording) instead of summing connected loads, which overstates what a service carries. basis = 125% of the recorded peak; total = basis + new load; headroom = service rating - total. A 200 A service with a 120 A recorded peak adding a 40 A EVSE is 1.25 x 120 + 40 = 190 A -> 10 A of headroom, fits with no upgrade; a busier 145 A peak is 221 A, over the 200 A service. VOID where the recorded data reflects on-site PV or peak-shaving that hides the true peak. A design aid; the AHJ and utility data govern.
Formula and source
basis = 1.25 x recorded_peak; total = basis + new_load; headroom = service_rating - total; fits = headroom >= 0 and not pv_or_peakshave.
The existing-load determination of NEC 2023 220.87 -- 125% of the maximum demand the utility metered over a year (or a 30-day recording) plus the new load -- by name. A computational aid; the AHJ-adopted NEC edition and the utility data govern.
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